At an electric power substation, for example, there is a technique for detecting malfunctions by comparing an electrical current value and a voltage value transmitted alternately from a protection relay. To perform the comparison accurately, accurate time synchronization is required.
As a conventional time synchronization technique, there is a time synchronization technique wherein an existing communication delay and an immediately previous communication delay are compared, and if an immediately previous inter-network communication delay is small, time synchronization is performed using time information when a communication delay is small.